1. Field of the Invention
Lens casting apparatus with particular reference to improvements in gaskets for lens casting cells.
2. Discussion of the Prior Art
Various synthetic resins have been used to make spectacles and instrument lenses, optical filters and the like. Lenses are traditionally cast between glass mold halves spaced apart by a T-shaped gasket and held with a spring clip as illustrated in U.S. Pat. No. 3,136,000, for example.
A casting precursor injected into the space between the mold halves is cured to a hardened state by exposing the filled cell to an elevated temperature for a proper period of time whereafter the lens is released from the cell. One effective lens is formed of a polymer produced from allyl diglycol carbonate, sold under the trademark CR39.
In view of the high shrinkage of optical casting resins during curing (approximately 13% for CR39), the casting cell gasket must be of sufficient resiliency to permit movement of the mold halves toward one another in the direction of lens thickness as resin shrinkage takes place. Shrinkage in directions normal to the direction of lens thickness, however, has heretofore presented the particular problem of premature release of lens material from the gasket walls which can cause distortion, cracking and other lens damage. This lack of proper adhesion of casting material to the gasket walls has seriously adversely affected acceptable product yield.
In light of the foregoing, it is an object of this invention to provide novel casting gasket geometry which will improve cast plastic lens yield and quality.
Another object is to provide a casting gasket geometry which, without appreciable added cost or complication of manufacture, offers unusual improvment in product yield and quality and greater than heretofore choice of gasket materials.
Still another object is to provide a casting gasket geometry which is readily adaptable to the casting of a wide variety of plastic compositions.
Other objects and advantages of the invention will become apparent from the following description.